The present invention is concerned with a novel process for the manufacture of a Wittig ester salt, especially of a (3-alkoxycarbonyl-2-butenyl)triphenylphosphonium chloride or ethyl sulphate, each of which is known as an important C.sub.5 building block for the manufacture of various polyenecarboxylic acid esters in the field of carotenoid chemistry.
The phosphonium salt 3-ethoxycarbonyl-2-butenyltriphenylphosphonium chloride can be manufactured readily, as is known, from ethyl .gamma.-chloro-tiglate by reaction with triphenylphosphine in toluene at about 110.degree.C. [N uirrenbach et al., Liebigs Ann. Chem. 1977, 1146-1159]. Ethyl .gamma.-chloro-tiglate itself and other lower alkyl .gamma.-halo-tiglates can be produced according to various known methods, for example:
from the alkyl tiglate by Ziegler bromination (using N-bromosuccinimide in carbon tetrachloride) [see Inhoffen et al., Liebigs Ann. Chem. 580, 1 (1953); Inhoffen et al., Liebigs Ann. Chem. 580, 7 (1953); as well as Korte et al., Chem. Ber. 89, 2675 (1956)]. However, Dreiding et al. [Helv. Chim. Acta 53,383 (1970); see also Cozzi et al, Tetr. Lett. 31, 5661 (1990)] were able to show later that this bromination does not proceed regioselectively and always gives a mixture of the two alkyl .gamma.-bromo-tiglate isomers (desired) and 2-alkoxycarbonyl-1-bromo-2-butene (byproduct), which are very difficult to separate from one another;
from an alkyl pyruvate by reaction with vinylmagnesium chloride (or with bromomagnesium acetylide and subsequent partial hydrogenation) followed by an allyl rearrangement with thionyl chloride (N urrenbach et al., ibid.) or a phosphorus trihalide [Kitahara et al., Tetrahedron 44, 4713 (1988)]. These methods are, however, expensive and uninteresting from the industrial point of view;
by means of a Wittig reaction of the triphenyl(Ph)phosphorane Ph.sub.3 P=C(CH.sub.3)COOC.sub.2 H.sub.5 with chloroacetaldehyde [Albertson et al., J. Med. Chem. 20, 602 (1977) as well as Stotter et al., Tetr. Lett. 1975, 1679]. This method is also expensive and uninteresting from the industrial point of view;
starting from 2-methyl-3-butenenitrile by ethanolysis, halogenation and subsequent dehydrohalogenation (German Offenlegungsschrift 3,244,273 as well as U.S. Pat. No. 4,937,308). This method has also been found to be relatively expensive and, moreover, gives unsatisfactory results; and
by reacting methyl vinyl ketone with hydrocyanic acid, subsequently converting the thus-obtained nitrile into an ester (Pinner reaction) and subjecting the latter to allyl rearrangement with thionyl chloride or another halogenating agent (German Offenlegungsschrift 2,852,343 and N urrenbach et al., ibid). The disadvantage of this method is, inter alia, the use of the highly toxic reagents hydrocyanic acid, thionyl chloride and methyl vinyl ketone.